#Define a procedure, factorial, that takes a natural number as its input, and
#outputs the number of ways to arrange the input number of itmes.

def factorial(n):
    if n == 0:
        return 1
    return n * factorial( n - 1 )    





print factorial(0)
#>>> 1
print factorial(5)
#>>> 120
print factorial(10)
#>>> 3628800